Fluorescent lamps generally consist of a tubular envelope with electrodes sealed at both ends with a coating of fluorescent material on the inner surface of the envelope and a mixture of vaporizable metal in an inert gas. A small quantity of electron emissive coating is applied to the surface of both electrodes in order to facilitate the emission of electrons. When a suitable voltage is applied across the fluorescent lamp to the electrodes the electrons which leave the electrodes consitute the electrical current through the lamp by the complex mechanism of ionization by collision. The electrons by the process of electron avalanche increasingly multiply to such an extent that the discharge current through the lamp is capable of excessively heating the electrodes to a point of destruction. A lamp current limiting device is therefore generally incorporated in the lamp operating circuit in order to ballast the current flow through the lamp to a reasonable value.
The operating life of the lamp is generally determined by the quantity of electron emissive coating available on the cathode emissive surface. For a fixed lamp operating voltage lamps are generally designed to be able to provide for both the emission of electrons from the cathodes and the generation of the above mentioned discharge current through the lamp. The electron emissive coating becomes depleted by successively starting and operating the lamp over a number of continuous cycles; or by the operation of a lamp current in excess of the design capabilities of the electrode structure. When the lamp is successively started over a substantial number of operative cycles the electrode coating material becomes dissipated by the local high energy fields which constitute the cathode fall region in the close vicinity of the cathodes. In the case of lamp operating currents in excess of the electrode coil design the cathode coating becomes depleted by the means of the excess operating temperatures of the cathodes causing the subsequent rapid evaporation of the emissive material.
In a well-designed lamp of the type having a ballast for providing the correct lamp operating current, and electrodes properly designed to meet the rated lamp life, lamp life then becomes a sensitive function of the quantity of electron emissive material present initially upon the electrodes so that lamp failure mainly occurs when the emissive coating becomes depleted from one or both electrodes. This invention therefore provides a cathode emissive coating material that results in large quantities of electron emissive material when applied to fluorescent lamp electrodes with correspondingly increased lamp operating life.